1. Field
The present application claims priority from Korean Patent Application No. 10-2006-102178, filed Oct. 20, 2006, the entire subject matter of which is incorporated herein by reference.
2. Background
Through mobile communication systems, the entire world may be connected at anytime with the use of a mobile terminal (i.e., a mobile station). In order to provide better communication services to more users by overcoming limitations of a frequency space, all areas covered by a central switching center may be divided into small serving areas called cells. A wireless base station, which is controlled by the central switching center, may be set up in each cell.
Handover allows the mobile station to continuously receive a currently used service when the mobile station moves between mobile communication networks. By performing the handover, communication channels between the mobile station and a base station, which provides a currently used service, can be changed automatically. In order to perform the handover, information on an identifier of a neighboring base station in a cell to which the mobile station will move may be necessary. The information may be included in a “handover require message.” The handover may begin when the mobile station sends the handover required message to the serving base station.
The mobile station may measure the power of the signals transmitted from the serving base station at a periodic interval and compare the measured power with a critical power. If the measured power is lower than the critical power, then average carrier-to-interference and noise ratios (CINRs) of the neighboring base stations and the serving base stations for a predetermined interval may be compared to each other. When the average CINR of the neighboring base station is higher than the CINR of the serving base station by a predetermined degree (i.e., by a hysteresis margin), the mobile station may decide to perform the handover to the neighboring base station, the average CINR of which is relatively high.
FIGS. 1 to 3 are graphs showing variation of CINRs versus time. As shown in FIG. 1, as a mobile station moves from a first cell of a serving base station to a second cell of a neighboring base station, an average CINR 11 of the serving base station may decrease while an average CINR 12 of the neighboring base station may increase. As for a predetermined interval t11, if the average CINR 12 is higher than the average CINR 11 by a hysteresis margin HM, then handover may be performed from the serving base station to the neighboring base station. When the mobile station moves from the first cell to the second cell only after repeatedly going in and out of the border of the first and second cells without moving directly from the first cell to the second cell as shown in FIG. 2, a plurality of handovers may be performed whenever an average CINR 22 of the neighboring base station is higher than an average CINR 21 of the serving base station by the hysteresis margin HM for predetermined intervals t21, t22 and t23. Even when the mobile station drops in the second cell for a moment and returns to the first cell as shown in FIG. 3, handovers may be performed only if an average CINR 32 of the neighboring station becomes higher than an average CINR 31 of the serving base station by the hysteresis margin HM for predetermined intervals t31 and t32.
Handovers may be preformed without considering frequent movements of mobile stations between a cell pair. Therefore, unnecessary handovers may be excessively performed. This may cause a ping-pong phenomenon of the handover, communication disconnection and increase of exchanging signal complexity and network load.